"HOLOTYPE—USNM 136939, gravid female, length 1.78 mm, some appendages and valves in alcohol, remaining appendages on slides.
TYPE-LOCALITY—Vema Cruise 17, station V-17-89A.
ETYMOLOGY—The specific name is derived from the Latin "evexus" [ = rounded at top] in reference to the more-or-less rounded tip of the rodshaped organ.
PARATYPES—USNM 136940, 136942, 136943, 136947, 136948; 5 gravid females. USNM 137031, 24 gravid females. Paratypes from same sample as holotype.
ADDITIONAL SPECIMENS—USNM 137054, 1 adult female without eggs; USNM 137055, 1 adult female without eggs, both from Vema Cruise 15, station V-15-110. USNM 137050, 1 adult female with unextruded eggs; USNM 137475 (shell only), both from Vema Cruise 15, station V-15-106.
DIAGNOSIS OF FEMALE—Carapace ornamentation similar to that on A. dithrix; carapace length 1.78 to 2.00 mm.
Mandible: Dorsal margin of basale with 4 bristles, 2 near middle, 2 terminal.
Seventh limb: Each limb with 10 bristles, 4 proximal, 6 distal; terminal comb with 14 alate teeth; no peg opposite comb.
Furca: Each lamella with 11-13 claws, 3 long claws followed by 8-10 short claws.
Rod-shaped organ: Tip rounded.
DESCRIPTION OF ADULT FEMALE—Surface reticulate with large pits and scattered single surface hairs; each valve with 4 distinct horizontal ribs similar to those on A. dithrix.
Infold: Infold on rostrum with 15 bristles; small bristle present on infold below inner margin of incisur; anteroventral margin with about18 short bristles; posteroventral and posterior list with numerous groups of 1 to 4 bristles; ridge with only 1 bristle present between posterior list and outer edge of valve; 1 bristle present near valve edge between ridge and posterior shell margin.
Selvage: Striate lamellar prolongation with short fringe discontinuous in area of incisur.
Size: USNM 136939, gravid female, length 1.78 mm, height 1.19 mm; USNM 136940, gravid female, length 1.87 mm, height 1.27 mm; USNM 136942, gravid female, length 1.85 mm, height 1.24 mm; USNM 136943, gravid female, length 1.83 mm, height 1.16 mm; USNM 136947, gravid female, length 1.83 mm, height 1.16 mm; USNM 136947, gravid female, length 1.89 mm, height 1.22 mm; USNM 136948, gravid female , length 1.96 mm, height 1.28 mm; USNM 137054, length 1.90 mm, height 1.23 mm; USNM 137055, length 2.00 mm, height 1.33 mm; USNM 137050, length 1.96 mm, height 1.36 mm.
First antenna: 1st joint with spines along ventral margin and on medial surface; 2nd joint with spines along dorsal and ventral margins and on lateral surface, and with 3 bristles, 1 ventral, 1 dorsal, 1 lateral (ventral and dorsal bristles with long proximal and short distal spines, lateral bristle with only short marginal spines); 3rd joint short with short spines along dorsal margin, and 1 ventral and 2 dorsal bristles, all with short marginal spines; 4th joint with spines along ventral and dorsal margins, and 6 spinous bristles, 4 ventral, 2 dorsal; sensory bristle of 5th joint with 6 proximal and 5 distal filaments including tip; medial bristle of 6th joint with 1 proximal wreath of long spines and short distal spines. Seventh joint: a-bristle similar to bristle on 6th joint; b-bristle slightly shorter than long bristles of 8th joint, with 2 proximal and 3 terminal filaments including tip; c-bristle long with 6 proximal and 3 or more terminal filaments. Eighth joint: d- and e-bristles bare, about same length as c-bristle; fbristle long with 5 proximal and 3 or more terminal filaments; g-bristle long with 4 proximal and 5 distal filaments including tip.
Second antenna: Protopodite with minute spines (not visible at low magnification) forming clusters on dorsal half of anterior part of medial surface. Endopodite 2-jointed: 1st joint with 6 bare bristles, 5 proximal, 1 distal; 2nd joint with 1 long spinous ventral bristle and 1 slender terminal bristle. Exopodite: 1st joint with short terminal medial spine with short marginal spines; bristles of 2nd and 3rd joints with proximal spines along ventral margin; bristle on 4th joint bare; bristles of joints 5 to 8 with natatory hairs; 9th joint with 7 bristles, smallest of these with short marginal spines, next with long proximal and short distal spines, remaining bristles with natatory hairs; distal margins of joints 2 to 8 with short spines forming row, additional cluster present near middle of 2nd joint; joints 3 to 8 with small basal spines.
Mandible: Coxale endite stout, bifurcate, with minute bristle near base. Basale: medial surfaces spinous with 6 bristles near ventral margin, 5 proximal, 1 near middle (3 proximal bristles pectinate); lateral surface spinous with 5 spinous bristles with bases near ventral margin; ventral margin with 3 spinous distal bristles; dorsal margin with 4 spinous bristles, 2 forming group near middle, 2 terminal (both terminal bristles and distal bristle in middle group with long proximal and short distal marginal spines, proximal bristle in middle group with only short marginal spines). Exopodite about three-fourths length of dorsal margin of 1st endopodite joint; end of exopodite hirsute and with stout spine having pronged tip; inner bristle with long proximal and short distal spines, outer bristle with short spines. Endopodite: 1st joint with 4 spinous ventral bristles; medial surface with spines forming clusters; ventral margin of 2nd joint with bristles forming 2 distal groups, of these, proximal group with 2 and distal group with 3 spinous bristles; dorsal margin with bristles forming 2 groups proximal to middle of margin, 3 bristles in proximal group, 5 or 6 in distal group; 1 short pectinate medial bristle present between 2 groups of bristles; medial surface with spines forming clusters; spines also present along ventral margin proximal to bristles; end joint with 3 claws and 4 bristles; dorsal claw about half length of medial claw; lateral claw longer than medial claw (lateral claw broken near tip on holotype); claws dentate along proximal ventral margin.
Maxilla: 1st endite with 9 spinous and pectinate terminal bristles; 2nd endite with 6 distal bristles; 3rd endite with 1 proximal and about 7 distal bristles; coxale with 1 stout plumose bristle; distal margin of basale with 3 bristles, anterior bristle shorter than others. Endopodite: 1st joint hirsute, with 1 a-bristle with long proximal and short distal spines and 3 6-bristles; end joint with 3 a-bristles, 2 b-bristles, c-bristles obscure on holotype, 3 d-bristles similar to those on A. dithrix (2 with stout marginal spines, 3rd ringed and with slender spines). Exopodite small with 3 bristles: proximal bristle short bare; middle terminal bristle long with long proximal and short distal spines; inner terminal bristle long with short marginal spines.
Fifth limb: Epipodial appendage with 52 bristles. Three endites present: 1st endite with 7 bristles, 2nd endite with 6, 3rd endite with 7. Exopodite: anterior margin of 1st joint with 2 spinous bristles near middle of distal margin and 1 short stout spinous bristle on small outer lobe; anterior surface of joint hirsute, and cluster of spines present on small outer lobe; main tooth consisting of 4 pectinate teeth; 1 spinous bristle present proximal to main tooth; triangular protuberance, anterior to main tooth, with small protuberance on proximal margin; posterior side of 2nd joint with total of 3 a- and b-bristles, and 1 long spinous c-bristle; margin of inner curvature of large tooth of 2nd joint with small indistinct node; outer lobe of 3rd joint with 2 spinous bristles, outer bristle less than half length of inner bristle; inner lobe of joint with 3 spinous or pectinate bristles; fused 4th and 5th joints hirsute and with 6 spinous bristles. (Morphology of 5th limb quite similar to that of A. dithrix.)
Sixth limb: 2 hirsute bristles present in place of epipodial appendage; endite I with 3 spinous bristles; endite II with 3 spinousbristles; endite III with 8 spinous bristles; endite IV with 7 spinous bristles; end joint prolonged posteriorly and with 19-21 spinous and plumose bristles; endites II to IV and end joint hirsute.
Seventh limb: Each limb with 10 bristles, 4 proximal (2 on each side) and 6 distal (3 on each side); each bristle with 3 to 6 bells and distal marginal spines; terminal comb with 14 alate teeth; no peg opposite comb.
Furca: Each lamella with 11 to 13 claws; claws arranged in 2 groups consisting of stout primary claws (claws 1 to 3) and short secondary claws (claws 4 to 11, 12, or 13) (holotype with 8 secondary claws on right lamella and 10 on left); claw 1 with teeth forming 2 rows, 1 lateral, 1 medial; claws 2 and 3 with teeth forming single row along posterior margin; both large and small teeth present in each row; secondary claws with minute teeth along posterior margin and hairs along anterior margin; hairs present on lamella following claws and at bases of main claws; spines present along anterior margin of lamellae.
Rod-shaped organ: Elongate with 2 indistinct sutures in broad middle part and tapering gradually to more-or-less rounded tip (under high magnification minute pointed process and hairs visible on tip); short surface hairs present on distal half.
Eyes: Lateral eye small with 2 ommatidia; medial eye pigmented, bare.
Eggs: USNM 136939, 7 eggs; USNM 136940, 3 eggs; USNM 136942, 2 eggs; USNM 136943, 3 eggs; USNM 136947, 10 eggs; USNM 136948, 8 eggs.
COMPARISONS—This new species very closely resembles A. dithrix, a species which also occurs at the type-locality of E. evexum. The range of carapace length of A. evexum (6 females) is 1.78 mm- 1.96 mm; the range of carapace length of A. dithrix (11 females) is 1.98-2.31 mm. The carapace of A. evexum is similar in size to that of A. chilensis and also A. pholion. Anarthron evexum differs from all previously described species in the genus by having 2 midbristles on the dorsal margin of the mandible. The distal half of the rod-shaped organs of previously described species in the genus taper at a fairly constant angle and terminate in a slender tip; whereas, the distal half of the rod-shaped organ of A. evexum tapers very slightly and terminates in a more-or-less rounded tip (under high magnification, a transparent pointed process is visible on the rounded tip). The new species also differs from A. dithrix in having numerous spines along the ventral margin of the 1st and 2nd joints of the 1st antenna.
DISTRIBUTION—This species was collected east of Argentina in both Subantarctic and Subantarcticto- 35°S regions (45°00'S to 54°10'S) at depths of 79 to 284 m." (Kornicker 1975, p.362-367)
